Many Worlds Interpretation existence

[tom.stoer]

The problems start, whenever you try to give more meaning to the quantum state then is implied by this minimal interpretation. Some think (in the past Einstein and Schrödinger were the most prominent physicists to do so) that this is not a complete description of nature since "in reality" (whatever "reality" means to them) all possible observables should have determined values always. It's not completely ruled out that maybe somebody one day finds some satisfactory theory, where this is the case, but Bell's work and the empirical precise findings with respect to it, imply that such a deterministic hidden-variable theory must be non-local, and so far there seems not to be a satisfactory such kind of theory in the relativistic realm.

That's why I think that the only mathematical object which can be "real" in the sense that it "faithfully represents external reality" is the state vector itself.

The definition of "external reality" is quite simple. This morning I got up and went to the bathroom. I believe the bathroom did exist all over night as part of "external reality"; it did not "become real" by being observed or used in the morning. Newton's equations of motion, conservation of energy etc. describing or predicting the continuously existence of the bathroom, and the bathroom itself have - for me - more than a pure epistemic meaning.

When talking about the quantum world we have to accept different laws of nature, but we need not give up this simple view on "external reality". Quantum mechanics is - in contrast to what many lectures and textbooks are trying to explain - perfectly consistent with this paradigm, once we accept that the mathematical entity we have to use as a "representation of external reality" is not a collection of classical properties like position, momentum etc. but the state vector itself.

In that sense Everett's approach, upgraded by decoherence, is a rather logical consequence of a simple philosophical position plus standard quantum mechanics. The only problem is to accept the weird consequences.

For me, all statements that quantum mechanics must not or cannot talk about "external reality" are fundamentally flawed. This is Bohr's legacy which Everett at al. try to overcome.

 

[martinbn]

Know ? I "know" it because I can compute any particle 4D path (and proper time along it), and pick the event pair whee both clock display the same time.No. The simultaneity of convention is when synchronized clock display the same time. There is no preferred frame involved here. Any frame will do.It means both measure can done at a perfectly valid absolute proper time. Simultaneity didn't disappear because different frame have a different perspective on some ordering. Events only happen once at one place, that's called causality.

Your terminology is very strange. For example, if we take my world line that of my great-great grandfather, when he was born his clock showed 0, when I was born so did mine. You would say that these two events a simultaneous, then when he was 10year, and when I was 10year, are two events that are simultaneous as well.

 

[LeandroMdO]

In that sense Everett's approach, upgraded by decoherence, is a rather logical consequence of a simple philosophical position plus standard quantum mechanics. The only problem is to accept the weird consequences.

The problem with Everettian accounts is not weirdness. I can accept weirdness, and so can most physicists. The problem is that at this moment the role of probability in the interpretation is still unclear. The interpretation can be kindasorta described in words, but no one has, to my knowledge, shown a convincing derivation that observers in this interpretation should split in such a way that, if they perform experiments, Born's rule is observed. Though it's been almost 30 years, the situation is not qualitatively different from the one described here (also see a small, more recent update here).

 

[tom.stoer]

The problem is that at this moment the role of probability in the [Everett] interpretation is still unclear ... no one has, to my knowledge, shown a convincing derivation that observers in this interpretation should split in such a way that, if they perform experiments, Born's rule is observed.

I agree in principle that there are open issues.

But from my perspective the arguments against Everett's approach are not more convincing than the arguments supporting his relative state interpretation. So for me it's still draw.

In addition it's not only about one interpretation but about a comparison of different interpretations. And here I must say that there are some rather ridiculous interpretations on the market. Really outstanding is the minimalistic interpretation saying that "everything works fine and we must not care about how and why". I clearly prefer an ambitious position with some open issues we can work on instead of no position with only open issues we should not even think about.

 

[SPW]

As scientists we need to keep philosophical interpretations of unexplained phenomena to a minimum. Keep an open mind, question everything & sooner or later, usually much later, the answers will come. "Betting" on the probability of an unquantifiable observation adds nothing to the proof. I suppose it puts me in the "shut up and calculate" club. As einstein once said "God does not play dice", by which he meant there must be something we don't yet know which lead to apparently paradoxical outcomes...

 

[tom.stoer]

As scientists we need to keep philosophical interpretations of unexplained phenomena to a minimum. Keep an open mind, question everything & sooner or later, usually much later, the answers will come. ... I suppose it puts me in the "shut up and calculate" club.

It doesn't b/c it seems that you are looking for explanations. :-)

 

[tom.stoer]

with Everettian accounts is not weirdness. I can accept weirdness, and so can most physicists. The problem is that at this moment the role of probability in the interpretation is still unclear. The interpretation can be kindasorta described in words, but no one has, to my knowledge, shown a convincing derivation that observers in this interpretation should split in such a way that, if they perform experiments, Born's rule is observed.

Though it's been almost 30 years, the situation is not qualitatively different from the one described here (also see a small, more recent update here).

Thanks for mentioning Kent.

Here's his introduction I fully agree with.

 

[vanhees71]

That's why I think that the only mathematical object which can be "real" in the sense that it "faithfully represents external reality" is the state vector itself.

The definition of "external reality" is quite simple. This morning I got up and went to the bathroom. I believe the bathroom did exist all over night as part of "external reality"; it did not "become real" by being observed or used in the morning. Newton's equations of motion, conservation of energy etc. describing or predicting the continuously existence of the bathroom, and the bathroom itself have - for me - more than a pure epistemic meaning.

When talking about the quantum world we have to accept different laws of nature, but we need not give up this simple view on "external reality". Quantum mechanics is - in contrast to what many lectures and textbooks are trying to explain - perfectly consistent with this paradigm, once we accept that the mathematical entity we have to use as a "representation of external reality" is not a collection of classical properties like position, momentum etc. but the state vector itself.

I couldn't agree more with this part of your posting. Indeed QT doesn't contradict this "simple philosophical position" at all. Only the notion of what a state is. In classical physics it's described by a point in phase space, in QT by the statistical operator of the system. The main difference between the classical description and quantum description is that in the classical case by the complete knowledge of the state one knows the values of all possible observables of this system (i.e., the values of all observables are always determined), while in QT only a well-defined class of observables take determined values, when any (pure or mixed) state of the system is prepared.

In that sense Everett's approach, upgraded by decoherence, is a rather logical consequence of a simple philosophical position plus standard quantum mechanics. The only problem is to accept the weird consequences.

For me, all statements that quantum mechanics must not or cannot talk about "external reality" are fundamentally flawed. This is Bohr's legacy which Everett at al. try to overcome.

I never understood the point of Everett's interpretation, let alone even in this modern form as "many-worlds interpretation" with the world branching into many parallel universes at any observation someone does on anything he or she observes, which also brings in the famous funny question by Bell, whether the observation of something by an amoeba is enough to cause a collapse (or in the MW interpretation the branching of the world) or whether you need some "more intelligent being" like a dog, monkey or human (who knows, how intelligent an amoeba maybe might be, but we just don't know it ;-)).

I think the logical consequence is the minimal interpretation, according to which QT is a formalism to describe probabilistically what we observe objectively in this real world, which you describe above. Indeed, as the name says, the quantum state describes the state of the system, and it's not a state vector but the statistical operator (or equivalently in the case of pure states, which are in general quite rare and must be carefully prepared in the lab, rays in Hilbert space).

 

[Boing3000]

Ah, ok. But then you have to deal with the twin paradox. You can separate your clocks and then bring them back together, and they won't read the same time--they will be sitting right next to each other, but reading different times. So which one is the "right" time--the time that is "absolute"?

This won't happens for photon, but for entangled electron, this would give an interesting setup, if half the electron are sent into an accelerator ring then slowed down for comparison (if entanglement could resit such a trip).
There is no problem for them sitting next to each other with at a different age. But then measuring the youngest batch before the oldest should lead to different result then the doing it the opposite.

 

[Boing3000]

Your terminology is very strange. For example, if we take my world line that of my great-great grandfather, when he was born his clock showed 0, when I was born so did mine. You would say that these two events a simultaneous, then when he was 10year, and when I was 10year, are two events that are simultaneous as well.

That would be, if both clock would have been synchronized. Have they ?
Events are absolute, and clock measure time. I had no idea why a relativist would have a problem with that...

 

[tom.stoer]

I couldn't agree more with this part of your posting. Indeed QT doesn't contradict this "simple philosophical position" at all. Only the notion of what a state is. In classical physics it's described by a point in phase space, in QT by the statistical operator of the system. The main difference between the classical description and quantum description is that in the classical case by the complete knowledge of the state one knows the values of all possible observables of this system (i.e., the values of all observables are always determined), while in QT only a well-defined class of observables take determined values, when any (pure or mixed) state of the system is prepared.

Find, thanks (but I am curious to understand how you can agree to my realist or even platonist position)

... let alone even in this modern form as "many-worlds interpretation" with the world branching into many parallel universes at any observation ...

This is just to increase circulation. I don't give a damn.

... which also brings in the famous funny question by Bell, whether the observation of something by an amoeba is enough to cause a collapse (or in the MW interpretation the branching of the world) or whether you need some "more intelligent being" like a dog, monkey or human (who knows, how intelligent an amoeba maybe might be, but we just don't know it ;-)).

That is an interesting question, yes.

I think the logical consequence is the minimal interpretation, according to which QT is a formalism to describe probabilistically what we observe objectively in this real world, which you describe above. Indeed, as the name says, the quantum state describes the state of the system, and it's not a state vector but the statistical operator (or equivalently in the case of pure states, which are in general quite rare and must be carefully prepared in the lab, rays in Hilbert space).

This is a possible consequence, but not necessarily a logical one. It simply denies the idea to let science provide realistic (ontic) explanations. Of course yours is one possible philosophical position, but there are others. They may not be yours, but they do not go away.

 

[name123]

This is a very good example for why I thik that quantum states are better interpreted in an epistemic sense and why the collapse hypothesis leads to problems with locality and causality.

My view is that the correlations, as all probabilistic relations of quantum systems, are described by the state of the system, and the state of the system is determined by preparation. The preparation in that case is when the entangled photon pair is created (e.g., by parametric downconversion by shooting a laser beam into a crystal). When A measures her photon's polarization state (her photon is defined by that it is registered by the detector at A's place), she immediately also knows the polarization state of B's photon (his photon is defined by that it is registered by the detector at B's place, which can be very far distant from A's detector). For B nothing has changed. He simply expects an unpolarized photon and gets with 50% probability the one or the other polarization when he measures it.

Let's now assume that A's detector is very close to the photon source, and B's very far, such that A measures her photon earlier than B. In other words, the measurement processes are assumed to happen as time-like separated events. Then "collapse" happens definitely at different times for A than for B: A changes the state of the photon pair due to her measurement result much earlier than B. Still, there is no contradiction by what's known to A and B concerning the outcome of their mesaurements. Both A's and B's photons are exactly unpolarized, i.e., the polarization state if maximally indetermined.

There's, of course, also no problem when the measurement processes are realized at space-like distances. Then you can always find a reference frame, where A and B register their result simultaneously or another reference frame, where A registers her result before B or again another frame, where B registers his result before A. Still there's no contradiction, because both, A and B always just find that their photons sent from the source of entangled photon pairs are precisely unpolarized.

When A and B compare their measurement protocols (always keeping detailed track about the time, when they registered their measurement outcome to be sure to relate always the pairs which where created together at the source), they find in any case the 100% correlations due to the preparation of the photon pairs in the polarization-entangled state.

Of course, here I made two assumptions: (a) the polarization measurements are local events as described by standard QED and thus the linked-cluster principle is valid, i.e., A's measurement cannot instantaneously affect B's photon and/or measurement apparatus (implied by microcausality) and (b) that all there is possible to be known about photons is what is described by quantum states, and since this is probabilistic knowledge (some may think only) it refers to ensembles of equally prepared quantum systems, i.e., the probabilistic information described by the prepared state can only be tested by collecting "enough statistics", i.e., using a sufficiently large ensemble.

The problems start, whenever you try to give more meaning to the quantum state then is implied by this minimal interpretation. Some think (in the past Einstein and Schrödinger were the most prominent physicists to do so) that this is not a complete description of nature since "in reality" (whatever "reality" means to them) all possible observables should have determined values always. It's not completely ruled out that maybe somebody one day finds some satisfactory theory, where this is the case, but Bell's work and the empirical precise findings with respect to it, imply that such a deterministic hidden-variable theory must be non-local, and so far there seems not to be a satisfactory such kind of theory in the relativistic realm.

I still do not understand your position.

Supposing C performs a Bell's Inequality Test-like experiment. Where 2 photons are entangled and then separated.

Do you agree with the following QM assumptions:
1) The preparation would determine the correlation between the two, but not the polarisation of A or B's photon which would both be undetermined.
?

Imagine that the polarisation of photon A is then measured at detector A. Do you agree with the following assumption:
2) The polarisation of A and B's photon is given by the measurement (because of the correlation determined by the preparation), so they can no longer be considered undetermined.
?

Note these assumptions do not involve consideration of whether the polarisation of B was ever measured by a detector B, and so do not consider whether there was a spacelike or timelike separation between those measurements. But if you agree with those assumptions, then there is the issue of how the measurement of A's polarisation could have determined the polarisation of B's photon (consider a spacelike separation example).

Though regarding your view, you mention that A and B will find the correlation determined by the preparation process. But the issue is not about the correlation, but the polarisation value of B's photon. You mention that with spacelike separation A will know the polarisation of B's photon, but for B things are unchanged (in terms of what B knows). That B expects the polarisation result to still be 50/50, but so what. Suppose B is in the same rest frame as A and they have clocks synchronised in that rest frame, and that B's measurement takes place a second after A's by their clocks (still spacelike separation just quite far apart). And that later they meet up, and compare notes. Are you suggesting that if you were B upon meeting back up, and seeing the log of A's measurements you would be claiming that a millisecond prior to you making the measurement a second later (in the frame of reference you were both in) the result was undetermined and it was 50/50 what the result would be because you did not know what A had found out?

Regarding your point about the problem brought up with the standard relativity interpretation, in which even with spooky action at a distance, A's measurement determined B's polarisation to the same extent that B's determined A's (or whether they happened simultaneously), because which happened first was truly relative . That is an interpretation dependent issue. As I mentioned earlier Relativity is compatible with Presentism (the view that the set of what exists only contains entities that exist in the present) where one frame of reference (presumably) corresponds to the present, but there would likely be no way of establishing experimentally which one. With a Presentist interpretation, the determination issue disappears. It allows for either A's to have determined B's or B's to have determined A's, even if there was no experimental way of telling which way around it was, or for them to have happened simultaneously .
P.S. I do not know what the banned LET thing is. Checked for "LET physics" and found https://en.wikipedia.org/wiki/Linear_energy_transfer, is that it? Or is it mentioning Presentism? If so I will avoid mentioning it again, though I would be interested in why it would be banned mentioning it, given that it is an interpretation that allows for a sequence of determination, and I am not sure why certain interpretations would be banned. Einstein seemed concerned with determination with his hidden variables vs dice idea, and some like MWI for its determination (though it has determination with no "spooky action at a distance"). If even asking about why it is banned is banned, could these last two paragraphs just be ignored.

 

[Lord Jestocost]

This is a very good example for why I thik that quantum states are better interpreted in an epistemic sense and why the collapse hypothesis leads to problems with locality and causality.

My view is that the correlations, as all probabilistic relations of quantum systems, are described by the state of the system, and the state of the system is determined by preparation. The preparation in that case is when the entangled photon pair is created (e.g., by parametric downconversion by shooting a laser beam into a crystal).

Let’s assume that all systems you have “prepared” are members of a pure ensemble – described by, maybe, one unique superposition state. Without assuming hidden variables, all of the systems in such a pure ensemble must be "physically" in the same state – otherwise whatever makes their states different would be a hidden variable, some "unknow preparation effect". Thus, when saying that a quantum state applies to ensembles and that the ensembles are not necessarily homogeneous, you have simply to answer the following physical question: “What differentiates the members of the ensemble from each other?” No answer means: Hiding a „hidden variable interpretation“ under the term „minimal interpretation“.

 

[tom.stoer]

No answer means: Hiding a „hidden variable interpretation“ under the term „minimal interpretation“.

I would not call this "hidden variable interpretation" but "agnostic interpretation". There are identical members in the ensemble behaving differently when being observed. A hidden variable theory would try to present some reason / mechanism / property / ... that would cause this difference. The minimalistic interpretation doesn't do this: there is no such mechanism, and one simply does not ask for such a mechanism.

 

[PeterDonis]

There is no problem for them sitting next to each other with at a different age.

Which means there can't be "absolute time" or "absolute simultaneity", as you were claiming before. Are you now retracting those claims?

measuring the youngest batch before the oldest should lead to different result then the doing it the opposite.

I don't think this is correct; measurements on the two entangled electrons should still commute.

 

[cosmik debris]

This morning I got up and went to the bathroom. I believe the bathroom did exist all over night as part of "external reality"; it did not "become real" by being observed or used in the morning. Newton's equations of motion, conservation of energy etc. describing or predicting the continuously existence of the bathroom, and the bathroom itself have - for me - more than a pure epistemic meaning.

As you get older you will visit the bathroom at more points during your sleep cycle verifying the continuing existence of the bathroom. This may have some connection with the Quantum Zeno effect.

Cheers

 

[Lord Jestocost]

I would not call this "hidden variable interpretation" but "agnostic interpretation". There are identical members in the ensemble behaving differently when being observed. A hidden variable theory would try to present some reason / mechanism / property / ... that would cause this difference. The minimalistic interpretation doesn't do this: there is no such mechanism, and one simply does not ask for such a mechanism.

In quantum mechanics, there is a fundamental difference between a “pure ensemble” and a “mixed ensemble”. It seems that the minimalistic interpretation secretly conceives a “pure ensemble” as some kind of “mixed ensemble” (“...there are identical members in the ensemble behaving differently when being observed...”). That’s why the minimalistic interpretation isn’t a true "agnostic" one. Therefore, why not asking for a mechanism which might be responsible for the different behavior of the ensemble’s members when being observed? Physics is about such questions! One can always stop talking (“…one simply does not ask for such a mechanism…“) to avoid to answer such questions, but that’s not the way to get a coherent physical formulation of a point of view.

 

[tom.stoer]

Of course you may ask these questions, but I think then you are no longer in-line with the advocates of the minimalistic interpretation who decided not to ask them.

 

[Boing3000]

Which means there can't be "absolute time" or "absolute simultaneity", as you were claiming before. Are you now retracting those claims?

There is no "claim" by defining absolute simultaneity by synchronized clock readings. I still don't understand your problem with that. In the context of one "thing" (an entangled state) evolution, it is hardly surprising. What really bothers you is that "thing" is at two place. What bothers you is non-locality.
I am not either going to retract that time is absolutely measured by clock, proper clock. It would be nice if you could stop pretend to contradict what the entire edifice of relativity stands on.

I don't think this is correct; measurements on the two entangled electrons should still commute.

It may not be, only an experiment could sort it out. It could very well assert that non-locality doesn't exist but only some other kind of spookiness (super determinism, or any other unrealistic proposition)

 

[Demystifier]

It's worth pointing out that lattice theories are defined in Euclidean space

They can also be defined in Minkowski space. But Euclidean space makes some numerical computations (like extraction of the ground-state energy) faster.

 

[vanhees71]

This is a possible consequence, but not necessarily a logical one. It simply denies the idea to let science provide realistic (ontic) explanations. Of course yours is one possible philosophical position, but there are others. They may not be yours, but they do not go away.

Perhaps, it's not a logical but rather an empirical consequence. As Einstein has suggested, one should look at what (theoretical) physicists do rather than listen to their words, and if you look at what physicists do with QT concerning describing and "understanding" their experimental findings is to use the formalism of QT with the minimal statistical interpretation.

 

[vanhees71]

They can also be defined in Minkowski space. But Euclidean space makes some numerical computations (like extraction of the ground-state energy) faster.

I'd say "possible" instead of "faster". A full lattice quantum QFT with real time would be a breakthrough at least for non-perturbative first-princile QCD simulations!

 

[Lord Jestocost]

As respects the "many world interpretation": Can someone tell me the preferred basis that correctly describes the universe and all the branching worlds within it or does all that hinge again on my conscious mind and its preferences?

 

[tom.stoer]

Perhaps, it's not a logical but rather an empirical consequence. As Einstein has suggested, one should look at what (theoretical) physicists do rather than listen to their words, and if you look at what physicists do with QT concerning describing and "understanding" their experimental findings is to use the formalism of QT with the minimal statistical interpretation.

But as you may know the same Einstein was absolutely not satisfied with "what theoretical physicists did with QM" :-)

I know that I cannot convince you to go beyond this minimalistic approach - and I'll respect this, of course. But at the same time I am asking you to respect other physicists who are interested in a realistic / ontic interpretation of QM and therefore do go beyond this minimalistic interpretation.

My emphasis on this point is not only b/c I am interested in these discussions.

My emphasis is due to the fact that the intention of Everett et al. can be understood in its entirety only if one understands their philosophical position; with a positivist or instrumentalist attitude their relative state approach is irrelevant or even waste. So one must not criticize their approach based on an instrumentalist point of view, that misses the point. There are two options: either one accepts their philosophical position (at least as a valid intellectual approach), then one can criticize their results in terms of physics; or one does not accept their position at all, then one should critize their position in terms of philosophical, but not the logical consequences.

For the latter discussion this forum is the wrong place. Therefore we should come to an agreement that for a discussion of the many worlds interpretation one should accept the philosophical position from which this interpretation emerged, and we should then discuss the physical consequences and obstacles, e.g. how can a probabilistic interpretation emerge? what would be a physically reasonable axiom which has to be added to the undisputable subset (Hilbert space, unitary time evolution)?

 

[tom.stoer]

As respects the "many world interpretation": Can someone tell me the preferred basis that correctly describes the universe and all the branching worlds within it or does all that hinge again on my conscious mind and its preferences?

The preferred basis is the basis that is selected by a measurement device. So if one measures the energy of a quantum object the preferred basis will be one that is "peaked" at the energy eigenstates.

We will check for some introductory papers.

 

[Lord Jestocost]

To my mind, as an experimental physicist, every interpretation of quantum theory is as good/bad as every other interpretation as long as it is incapable to predict physical consequences on the basis of which its truthfulness can be proven. All adherents of all interpretations should understand that these interpretations are to a large extent based on personal psychological predispositions. It makes no sense trying to pick other interpretations into pieces (this reminds of religious disputes); a scientific approach means to think of or to provide experimental consequences that might “prove” that one’s own interpretation might be closer to the truth.

 

[Boing3000]

and we should then discuss the physical consequences and obstacles

I am thinking of a Schrödinger's cat type of situation. If I can understand that in the Hilbert space, can cohabit various orthogonal'ized versions of the cat, in various state of alive, rotten, or dying. If the Hilbert space is still that real "space" that can be projected in any "true" 3+1 dimension, I don't think any physical consequence could be "tested", aside that a dead cat would lie more "down" the box. That mean that across universes, there would be more gravity "down the box" if the likeliness of dead cats were bigger...(even if the cat is alive).

I know that QM don't pretend to account for gravity, but I think it is a logical obstacle for MWI. For example if I propose to boost the cat's thought experiment.
Let's say the radioactive decay trigger the engine of a 10 tons rocket that will or not fall onto a neutron start hovering 10 ton below the black hole limit.
Do a pure MWI like Hilbert space can still describe uniformly all universes ? Aren't there in-reconcilable "coordinate" holes in some of them ?

A simpler way would be to ask if it is even possible to talk of "universe" and not only a very small subset of it (like a cat's box) ?

 

[tom.stoer]

A scientific approach means to think of or to provide experimental consequences that might “prove” that one’s own interpretation might be closer to the truth.

Yes - but a scientific approach is not restricted to experimental consequences, it also aims for reasonable theoretical explanations.

 

[Lord Jestocost]

The preferred basis is the basis that is selected by a measurement device. So if one measures the energy of a quantum object the preferred basis will be one that is "peaked" at the energy eigenstates.

We will check for some introductory papers.

That means, I know when using a certain measurement device that every outcome - after selecting the preferred basis - will actually occur (according to MWI). Why do I then always have the "feeling" that the results of an intended measurement are uncertain? What branches at the end? An “objective universe” (where I exist) or my "ideational realization" or "ideational construction" of something which I conceive as "universe"?

What even Everett didn't grasp: There are no means to re-establish Einstein's comfortable and deterministic classical world.

Edit: What Everett didn't also understand: You are sitting in some arbitrary branch an think that your thinking within this branch is of any relevance for an overall understanding.

 

[AlexCaledin]

All adherents of all interpretations should understand that these interpretations are to a large extent based on personal psychological predispositions.

- but it's probably only Niels Bohr, whose predisposition was to avoid every sort of wishful thinking . . .